Light detection and ranging (lidar) and laser detection and ranging (ladar) systems, hereafter collectively referred to as ladar systems, are remote sensing systems that send light from a transmitter to a target, detect light scattered from the target using a receiver, and infer properties about the target from the detected light. Inferred properties may include range, velocity, vibration, material, and other parameters. Generally these systems can be divided into two categories: direct detection ladar and coherent detection ladar, depending upon the method used for detection of the scattered light.
Direct detection systems are generally simpler to implement since the receiver only requires a detector that converts absorbed photons into electrical current and electronic components that amplify and filter the resulting electrical signal for further processing. Coherent detection systems are generally more complex to fabricate, in that the receiver must incorporate a local oscillator (LO) beam of light that is mixed with the received light to produce a signal proportional to the coherent addition of the local oscillator (LO) electric field and the signal field.
Historically, one or the other type of detection has been designed into ladar system architectures, primarily as a result of a priori decisions about what parameters the system must detect. All parameters can in principle be detected with coherent detection, whereas direct detection systems cannot make measurements that require retrieval of the signal optical phase. Important cases that involve phase retrieval include Doppler measurements of speed, velocity, and vibrations. It has generally also been the case that coherent detection systems provide greater sensitivity than direct detection systems. Appropriately designed coherent detection systems can detect the reception of single photons, whereas many direct detection systems have noise that limits the detection sensitivity to far higher levels.
Many ladar systems that operate in the field, for example, from aircraft, are severely limited in the amount of space they can occupy and the electrical power they can draw from the aircraft. In addition, operation with the greatest possible sensitivity is essential. Furthermore, operational scenarios in current and future systems place a high premium on multi-functionality, such that it is highly desirable for a single installed system to be capable of carrying out multiple measurements with high efficiency. This is particularly the case where size, weight, and power (SWaP) are at a premium, as is the case in land-based vehicles, aircraft, aircraft pods, UAVs (unmanned aerial vehicles), and MAVs (micro air vehicles). This trend to improved SWaP is anticipated to continue, resulting in a desire to increase the multi-functionality of measurement systems. It is also highly desired for ladar systems to be immune to jamming and background noise. Since direct detection receivers are sensitive to any photons of the correct wavelength that are absorbed by the photo detector, they are relatively sensitive to interference. Coherent detection systems, on the other hand, are only sensitive to photons that, when mixed with the LO, produce a beat frequency within the RF bandwidth of the receiver, and in addition, are propagating in the same single spatial mode as signal photons. For this reason coherent detection systems are generally many orders of magnitude less sensitive to interference.